Answer:
-22 or -2
Explanation:
Understanding size
If RS is 10, it does not matter where they are. As long as they are 10 units apart, it works.
However, we already know S lies at -12 so we only have 2 choices for R.
Check the picture I uploaded for better understanding.
This is because RS doesn't mean that R has to be before S. It just means that they are a segment. This means that RS and SR are the same segments.